This invention is a method for merging different raster images, representing contone data (continuous-tone pictures) and line art (text and graphics), into a single multi-level or grayscale raster image suitable for driving a grayscale printer or other output raster device, or a binary raster printer via a halftone generator.
A potentially useful feature of a printer would be the ability to print text overlaid on a scanned picture. Printer controllers are normally designed for printing one or the other. A printer controller for text would send a high-resolution bitmap to the print engine which prints the output hard copy, while a controller for scanned pictures would send a medium-resolution byte map which a halftone generator would convert into a high resolution bitmap for driving a binary print engine.
When text and scanned pictures are combined, the controller would send the common denominator to the print engine in the form of a high-resolution byte map. In other words, if the text is being received at 300 bits per inch and the pictures are being received at 100 pixels per inch and 8 bits per pixel, a final raster resolution of 300 pixels per inch, 8 bits per pixel, would accommodate both inputs. In a typical system, a page buffer with the capacity to store a page of raster data at the final raster resolution is loaded with text and pictures, and then the raster data are sent to the print engine line by line from the buffer. However, this system of converting data to a common form before merging them is not efficient in terms of handling text where there is a requirement for high resolution at the edges of the characters, but where the color either seldom changes, or is constant. Similarly, a byte map is fine for scanned pictures, but high-spatial resolution is not required. The basic problem, therefore, is how to efficiently handle text and graphics at one spatial resolution, and scanned pictures at another, to produce a final print containing both.
In the prior art, graphic arts applications have already had to deal with electronically merging scanned pictures, text and line art, and methods have been described that combine high-resolution binary or line art data and medium-resolution multi-bit picture data.
In Hell graphic arts film plotters (ca. 1982), raster data is sent to the halftone generator and output scanner as a sequence of bytes. A bit in each byte identifies six of the remaining bits as either contone data to be sent to the output scanner via the halftone generator, or as binary data, packed six bits to a byte, that is to bypass the halftone generator and be sent directly to the output scanner. The spatial resolution of the binary data is six times the spatial resolution of the contone data.
A similar approach was taken in an MIT-built computer controller for engraving gravure cylinders [D.E. Troxel et al., IEEE Trans. Systems, Man and Cybernetics, Vol. SMC-11, No. 9, September 1981, pp. 585-596]. In a sequence of bytes, graphics data (text and line art) was stored as two 4-bit samples per byte, while contone data was stored as one 8-bit sample per byte. In both cases, one byte of data was used to engrave one gravure cell. The spatial resolution of the binary data is twice the spatial resolution of the contone data. A 4-bit value and and 8-bit value were reserved as codes to switch between graphics and contone data in the byte sequence.
In current standards for the interchange of graphic arts data, there are separate standards for line art data and for continuous-tone pictures. A line art image consists of a rectangular array of picture elements, each of which holds a limited number of colors defined in a color table [ANSI IT8.2-1988, User Exchange Format for the Exchange of Line Art Data between Electronic Prepress Systems via Magnetic Tape]. Because a line art image typically has continuous areas of many pixels of the same color (and the same color table entry), it is amenable to run-length coding. One of the color table entries is reserved for a transparent color. A transparent color is used to distinguish a run where no color is present and the underlying color (if any) is allowed to show through.
In files which contain both line art and contone picture data, the line art data and the contone picture data are represented separately [ANSI IT8.4-1990, Device Exchange Format for the On-Line Transfer of Color Proofs from Electronic Prepress Systems to Direct Digital Color Proofing Systems]. Where they overlap, the line art data takes precedence. For the picture data to be visible when overlaid with a line art data, the line art data must be transparent (see FIG. 5A). In this format, the line art and pictures must overlay exactly, that is, cover the same area on the page or image.
A similar approach is described in documentation for the Canon Color Laser Copier Intelligent Processing Unit (IPU) [Intelligent Processing Unit Series, Service Manual, Revision 0, April 1989; Canon Color Laser Copier 500 IPU Programming Manual, Draft, Aug. 28, 1989] (see FIG 5B). The IPU can store contone pictures, a color palette or table, and a binary image. The binary image controls how the contone pictures and the colors defined in the table are combined. Where the binary image has value 1, a color specified by the color table is output. Where the binary image has value 0, corresponding to "transparent," the underlying picture is output.